Uranium occurs in low concentrations in various ores and mineral deposits. Although the concentrations are often too low to justify mining the deposits just for the uranium, when the deposit is being mined already it is often economical to recover the uranium as well. For example, phosphate deposits in Florida and copper deposits in Utah contain a small amount of uranium.
In a process developed by Hurst at Oak Ridge National Laboratories the uranium in wet-process phosphoric acid can be recovered by extraction with an organic solvent, namely, di(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene, the effectiveness of which is enhanced with the synergistic agent tri-n-octyl phosphine oxide (TOPO). The organic extract is stripped with a concentrated ammonium carbonate solution (1.5-2.0 M) which causes the uranium to precipitate as ammonium uranyl tricarbonate (AUT).
Unfortunately, iron is also present in the phosphoric acid and it too is extracted and stripped and precipitated with the uranium. Thus, the product may contain as much as 2 parts (by weight) of iron per 100 parts of uranium. This high iron contamination is undesirable because it interferes with the enrichment of the uranium (i.e., processes for increasing the proportion of the U.sup.235 isotope).